The present invention relates to a sheet convey apparatus for conveying sheets one by one in the apparatus as in a copying machine, facsimile apparatus, or a certain type of scanner and, more particularly, to a sheet convey apparatus having a mechanism for preventing overlapping feed of a paper sheet.
In general, in an apparatus such as a document reading apparatus which reads a document, a printer or copying machine which prints an image, or the like, a plurality of sheet-type documents or a plurality of recording sheets are set on a tray. Image reading or printing is performed on the sheets one by one. In a sheet convey apparatus in which a paper sheet is conveyed in this manner, the plurality of sheets set on the tray must be separated one by one reliably and be fed into the apparatus. If overlapping sheets are fed, some document may be left unread, or an inconvenience may occur during conveyance, and printing is not performed well. For this reason, an overlapping feed preventive device is attached to the sheet convey apparatus.
As the overlapping feed preventive device, a comparatively large one is available which includes a mechanism to chuck the stacked sheets one by one from the uppermost layer and feed them into the apparatus. Generally, a device is used which feeds the uppermost sheet by considering the rotating directions of a pair of feed rollers.
As shown in FIG. 8, a conventional sheet convey apparatus 300 has a main feed roller 303 which applies a conveying force in a convey direction 302 to stacked sheets 301, and a retard roller (or separation roller) 304 which applies a conveying force in a direction opposite to the convey direction 302. When the sheets 301 including two stacked sheets as shown in FIG. 8 enter between the main feed roller 303 and retard roller 304, the main feed roller 303 applies a frictional force in the convey direction 302 to a first uppermost sheet 3011, and the retard roller 304 applies a frictional force in a direction opposite to the convey direction 302 to a second uppermost sheet 3012. If the frictional force between the sheets 3011 and 3012 is sufficiently smaller than the frictional force between the two rollers 303 and 304, the first uppermost sheet 301, is conveyed in the convey direction 302. Simultaneously, the second uppermost sheet 3012 receives from the retard roller 304 a force to push it back in the direction opposite to the convey direction 302. Consequently, only the first uppermost sheet 3011 is fed into the apparatus, and overlapping feed is prevented.
Assume a case wherein the frictional force of the retard roller 304 with respect to the sheet 301 is larger than that of the main feed roller 303. This is based on the premise that the frictional coefficients of the two surfaces of the sheet 301 are equal. On this premise, when only one sheet 301 enters between the main feed roller 303 and retard roller 304, the convey force in the direction opposite to the convey direction 302 which is caused by the friction of the retard roller 304 becomes larger than the convey force in the convey direction 302 which is caused by the friction of the main feed roller 303. Therefore, in this case, the sheet 301 cannot be conveyed in the convey direction 302.
In order to solve this problem, it has been proposed to attach a torque limiter to a retard roller, as shown in Japanese Patent Laid-Open No. 9-235036, to limit the driving force of the retard roller.
When the torque limiter is attached to the retard roller, the press by the main feed roller 303 and retard roller 304 and the torque of the torque limiter must satisfy both the first and second conditions to be described hereinafter. This will be explained with reference to FIG. 8.
The first condition applies when one sheet 301 is present between the main feed roller 303 and retard roller 304. According to the first condition, the retard roller 304 must rotate in a direction (counterclockwise) to convey the sheet 301 in the convey direction 302, so that the sheet 301 is conveyed in the convey direction 302 stably. More specifically, while a force to rotate the retard roller 304 in the direction (clockwise) indicated by an arrow is transmitted to the retard roller 304, the retard roller 304 must be rotated by the main feed roller 303 in a direction opposite to the transmitted force.
The second condition applies when two or more sheets 301 are present between the main feed roller 303 and retard roller 304. According to the second condition, the retard roller 304 must rotate in the direction (clockwise) indicated by the arrow so as to push back all the sheets 3012 except for the first uppermost sheet 301, which is in contact with the main feed roller 303.
When a torque limiter is to be attached to the retard roller 304, its torque must satisfy the first and second conditions. If the torque of the torque limiter is excessively large or the pressing force between the main feed roller 303 and retard roller 304 is small, the first condition cannot be satisfied. If the torque of the torque limiter is excessively small or the pressing force between the main feed roller 303 and retard roller 304 is large, the second condition cannot be satisfied.
The torque of the torque limiter does not always maintain a constant value, but changes to a certain degree. On the sheet convey apparatus side, the degree of wear of the retard roller 304 and the state of the sheets 301 change. Consequently, while the sheet convey apparatus functions well at a certain time with respect to the sheets 301 in various types of states, overlapping feed may occur at another time, or one sheet 301 cannot be conveyed well.